Restrictions on rebooking after part of itinerary cancelled?

Hi everyone. I was booked for Polaris Bangkok —> Hong Kong —> San Francisco. I checked the United app this morning and saw that my flight from Hong Kong to SFO was cancelled. My new itinerary had a 17 hour layover, and then I was booked on a flight to Japan on a partner airlines in economy, and then another partner flight in economy to SFO. Total travel time went from ~23 hours to ~48 hours. I never received an email about the change.

When I called United (Premier Desk), they told me the plane had a mechanical issue, so the flight was cancelled. I looked up multiple other United itineraries from Bangkok to SFO on Polaris with plenty of seat availability. I specifically asked if I could be routed through Taipei. The agent told me that she could only rebook me on travel through Hong Kong, and that if I wanted to connect through a different city, I would have to cancel my itinerary and rebook at the current price.

Ultimately I canceled the entire itinerary because it was totally insane and I needed to get home. I received a full refund and booked a last minute ticket on Starlux. United offered a $650 flight credit for the trouble.

A few questions:

1. How restricted are agents in their ability to rebook you when flights are cancelled due to mechanical issues? I was very confused that they could not put me on an itinerary that got me to SFO not far off the originally scheduled arrival time. They just kept saying that I had to go through Hong Kong and that limited what they could do for me, and resulted in a drop from Polaris to Economy for most of the flight times.

2. Should I ask for more compensation than a $650 flight credit? Do they ever offer a cash payment vs a credit?

Thank you, everyone. Overall, I’m grateful to be getting home at the same time I was originally supposed to.